1. Technical Field
This invention relates to the field of transmitting high definition television signals and, more particularly, to a method and apparatus for increasing the definition of an N.T.S.C. signal using a separately generated and transmitted augmentation channel.
2. Description of the Related Art
There is a growing interest in the transmission of television signals which allow an increase in picture definition in both the vertical and horizontal dimensions. There are, however, many millions of television sets already in use which receive and display the standard North American and Japanese television transmission standard (known as the National Television Subcommittee or N.T.S.C. standard).
Since there are so many existent television sets which display only an N.T.S.C. television signal, there have evolved two basic approaches to providing high definition television service. One approach is to transmit a high definition television signal in a new format and, consequently, impose a requirement on all users that they either obtain a decoder for their existing television sets capable of recovering an NTSC signal from the new format or buy a new television receiver for receiving the new format. To receive the new format and to display a high resolution image would require the user's obtaining a new expensive high definition television receiver. To make the alternative decoder viable, an obvious objective of such an approach is to make the cost of the NTSC decoder for an existing television as inexpensive as possible. This approach, however, is highly unsatisfactory to broadcasters who may lose considerable revenue from users and at the same time must expend considerable sums to adapt to the new format.
One solution which follows this approach relates to the transmission of a high definition multiplexed analog components (MAC) signal from which an NTSC signal may be inexpensively derived. The solution is described in copending U.S. application Ser. No. 092,305 filed Sept. 2, 1987, incorporated herein by reference. In the disclosed technique a line summation signal is generated from which improved horizontal resolution information may be derived and a line difference signal is generated from which improved vertical resolution information may be obtained. A two dimensional diagonal filter is applied to a high resolution signal and alternate samples dropped leaving a quincunx or figure of five pattern. A full high resolution signal is restored by reintroducing samples previously dropped by interpolation from transmitted line summation and line difference signals.
Any new format for the transmission of higher definition signals should take into account the existing investment by the public and by manufacturers in existent NTSC television sets. For example, if a television channel distributed by a broadcaster in NTSC format were to change to a new television signal format, transmission of the NTSC format signal could not be practically terminated unless the new format could be received by existing standard NTSC compatible sets. Otherwise, the broadcaster would immediately be deprived of all of its viewers, and their viewers deprived of service until each viewer upgrades their home equipment to receive the new standard format.
For this reason, there is considerable interest in an alternative approach to the development of a high definition television (HDTV) format which may be received and displayed (without increased resolution) by existing television receivers. New receivers, containing more advanced circuitry, should receive and display high definition pictures when tuned to the same transmitted channel. A television signal format with the property of being compatible with existing NTSC and new high definition television sets is known as an "NTSC compatible HDTV signal".
One way to create an NTSC compatible HDTV signal is to introduce an "augmentation channel" to the present NTSC signal. A wide picture format television transmission system involving an augmentation channel is proposed in U.S. Pat. No. 4,581,640 to Cole, incorporated herein by reference. A second separate channel to the NTSC signal channel augments the existing NTSC signal by carrying sidepanels to increase the width of the picture and additional information to increase the vertical definition and the horizontal definition. As developed and proposed for implementation by North American Phillps Corporation and in accordance with a convention of reference to the output scanning rate, the augmentation channel is formed by time-expanding high frequency signals by 2:3 and modulating a line difference signal to a subcarrier at 12.2 megahertz, a multiple of the NTSC line frequency and above the time expanded high frequency Y.sub.H luminance component of the high definition NTSC signal. The line difference component then is multiplexed at approximately 9.4 to 13 megahertz while the high definition information is at 7.3 to 10.4 megahertz.
This multiplexing takes advantage of the little overlap (9.4-10.4 megahertz) between the two components. An improvement to the initial Philips proposal is described in Tsinberg, U.S. Pat. No. 4,694,338, also incorporated herein by reference, who proposes a solution to problems created by overlapping portions of the transmitted augmentation channel. Any claim to improved horizontal definition appears to be related to the line difference signal containing horizontal information. However, while the Philips augmentation channel proposal is viable, there is still a requirement in the art for an economical and efficient alternative design for implementation of the augmentation channel and which further improves horizontal resolution.